Fundamental physics and cosmology using astronomical laser frequency combs

This thesis presents advances in procedures for wavelength calibration of astronomical spectrographs. The context of this research is a search for new physics using astronomical observations of quasars. The aim is to reach the highest possible accuracy of wavelength measurements using laser frequency comb technology. Very accurate wavelength measurements are important for several scientific projects motivating the construction of large optical astronomical facilities, such as the Extremely Large Telescope. In many extensions of General Relativity, the values of fundamental constants, such as the fine structure constant (α), change. This effect is measurable, as a change in α perturbs the energy levels of atomic and molecular transitions. Cosmological changes in the value of α can be probed by measuring small wavelength shifts of transitions in quasar spectra. Previous searches for such an effect found evidence for a dipole variation of α across the sky with ≈ 4σ significance. If proven to be correct, this result could possibly pave the way to a Grand Unified Theory. It is therefore imperative to remove all systematic effects which may spoil α measurements. One such effect pertains the wavelength calibration of astronomical spectrographs. Here, I present research performed using the first generation of astronomical laser frequency combs (LFC) on one of the most stable astronomical spectrographs: the High Accuracy Radial-velocity Planetary Searcher (HARPS). The results are highly encouraging; the LFCs provide unprecedented wavelength calibration accuracy. The numerous and densely spaced LFCs lines allow practically all wavelength calibration effects to be removed, starting a new era for α measurements at high redshift. The LFC lines also allowed detector characteristics to be measured in a way that was not possible before. Applying these advanced wavelength calibration methods to HARPS observations of the quasar HE0515-4414 allowed us to constrain variations in α in an absorption system at zabs = 1.15 seen towards this quasar. We obtained 40 measurements of the fractional change in α in this system, aided by artificial intelligence methods. The average of the measurements is Δα/α = -0.27 ± 2.41 × 10-6, consistent with the prediction of the dipole for the sky position of this quasar but also consistent with zero change. The large number of measurements in this system allowed us to constrain small-scale variations of α for the first time: Δα/α % 9×10-5 across ≈ 25 kpc scales.